Substitutions of individual nucleotides in the DNA sequence, sequence differences in one nucleotide. For example, the sequences AAGCCTA and AAGCTTA differ in that cytosine (C) has been replaced by thymine (T).
SNPs are found both within the coding sequences of genes and in non-coding regions. There are synonymous and non-synonymous SNPs: synonymous ones do not affect the amino acid sequence of the protein, but non-synonymous ones change it.
There are several databases of human SNP. They are used in genetic mapping and in genome-wide association studies (GWAS). Some of the human SNPs have been shown to be associated with certain diseases, but most, apparently, are selectively neutral. SNPs, along with other genetic markers (STRs), are used to build phylogenetic trees.